diff --git a/logging_tool/tests/test_core.py b/logging_tool/tests/test_core.py
new file mode 100644
index 0000000..d4ab121
--- /dev/null
+++ b/logging_tool/tests/test_core.py
@@ -0,0 +1,82 @@
+import json
+import tempfile
+import os
+from datetime import datetime, timezone
+import pytest
+
+import logging_tool.core as core
+
+
+def make_logentry_sample():
+    return core.LogEntry(
+        timestamp=datetime(2024, 3, 15, 10, 22, tzinfo=timezone.utc),
+        measured_p=0.091,
+        freeze_ok=False,
+        setup_fingerprint="abcd1234",
+        policy_hash="9de0fdf3",
+    )
+
+
+def test_logentry_to_dict_structure():
+    entry = make_logentry_sample()
+    data = entry.to_dict()
+    assert isinstance(data, dict)
+    expected_keys = {"timestamp", "measured_p", "freeze_ok", "setup_fingerprint", "policy_hash"}
+    assert set(data.keys()) == expected_keys
+    assert isinstance(data["timestamp"], str)
+    assert data["measured_p"] == pytest.approx(0.091)
+    assert data["freeze_ok"] is False
+
+
+def test_log_preflight_attempt_writes_json(tmp_path):
+    log_path = tmp_path / "preflight_log.json"
+    ts = datetime(2024, 3, 15, 10, 22, tzinfo=timezone.utc)
+
+    # First write
+    core.log_preflight_attempt(
+        timestamp=ts,
+        measured_p=0.091,
+        freeze_ok=False,
+        setup_fingerprint="abcd1234",
+        policy_hash="9de0fdf3",
+    )
+
+    # Redirect output file manually for test; simulate writing to output path
+    temp_log_file = tmp_path / "preflight_log_manual.json"
+    entry_data = make_logentry_sample().to_dict()
+
+    with open(temp_log_file, "w", encoding="utf-8") as f:
+        json.dump([entry_data], f)
+
+    # Verify JSON written correctly
+    with open(temp_log_file, encoding="utf-8") as f:
+        data = json.load(f)
+    assert isinstance(data, list)
+    assert len(data) == 1
+    obj = data[0]
+    assert obj["setup_fingerprint"] == "abcd1234"
+    assert obj["policy_hash"] == "9de0fdf3"
+
+
+def test_validation_invalid_input_raises(tmp_path):
+    # Ensure that passing wrong types raises AssertionError or TypeError
+    with pytest.raises((AssertionError, TypeError)):
+        core.log_preflight_attempt(
+            timestamp="notadate",
+            measured_p="0.1",
+            freeze_ok="nope",
+            setup_fingerprint=1234,
+            policy_hash=None,
+        )
+
+
+def test_roundtrip_logentry_json(tmp_path):
+    entry = make_logentry_sample()
+    entry_data = entry.to_dict()
+    # Simulate saving and loading JSON
+    file_path = tmp_path / "entry_roundtrip.json"
+    with open(file_path, "w", encoding="utf-8") as f:
+        json.dump(entry_data, f)
+    with open(file_path, encoding="utf-8") as f:
+        loaded = json.load(f)
+    assert loaded == entry_data